var portfinder = require('portfinder');

  portfinder.getPort(function (err, port) {
    //
    // `port` is guaranteed to be a free port
    // in this scope.
    //
  });

  const portfinder = require('portfinder');

  portfinder.getPortPromise()
    .then((port) => {
        //
        // `port` is guaranteed to be a free port
        // in this scope.
        //
    })
    .catch((err) => {
        //
        // Could not get a free port, `err` contains the reason.
        //
    });